1. Field of the Invention
The present invention relates to an organic thin film electroluminescence device and a process for production thereof, and more particularly to an organic thin film electroluminescence device having heat resistance and a long life and a process for producing an organic thin film electroluminescence device in which patterning can be conducted by photoetching.
2. Detailed Description of Related Arts
An electroluminescence device (hereinafter referred to as an "EL" device) is easily distinguished because it emits light, and is durable to impacts because it is a completely solid device. Recently various EL devices using inorganic or organic compounds in its light emitting layer have been proposed and have been attempted to be put into practical use. Of these, an organic thin film EL device can greatly decrease an applied voltage and, therefore, various materials have been developed therefor.
For example, an organic EL device comprising anode/triphenylamine derivative/light emitting layer/cathode as described in Japanese Patent Application Laid-Open No. 194393/1984 is excellent in low applied voltage, multicolor display, high brightness and high efficiency. However, a device in which 8-hydroxyquinone aluminum complex is used in the light emitting layer has an obstacle in its practical use, because its service life, when it is driven at a brightness of 50 cd/m.sup.2, is about several hundred hours.
One of the causes is heat deterioration. The above 8-hydroxyquinone-aluminum complex is a compound having a relatively high heat resistance, but inevitably suffers from heat deterioration.
In recent years, various devices in which an organic low molecular weight vacuum deposited film is used as the light emitting layer have been produced. For example, an EL device comprising anode/styrylamine light emitting layer/charge injection layer/cathode is known (Appl. Phys. Lett., 55 (1989)1489). Many of the organic low molecular weight compounds to be used in the organic low molecular weight vacuum deposited film have melting points of 300.degree. C. or lower, and even in the form of amorphous film, its glass transition temperature is lower than 100.degree. C. In any case, there is a problem of heat resistance, and heat deterioration inevitably occurs.
Although the behavior of the above heat deterioration has not been examined in detail yet, it is considered that crystallization and melting of the light emitting layer, enlargement of pinholes, peeling off of the light emitting layer and so forth, all caused by heat generation of the device due to Joule heat, are responsible for the heat deterioration.
Many light emitting layers using organic low molecular weight compounds are soluble in a solvent, and even if insoluble in a solvent, they are deteriorated by peeling off due to the solvent. Thus photoetching cannot be applied. Accordingly patterning is difficult, and this becomes an obstacle in production of displays and so forth in the form of EL device.
It is reported that a device in which a thin film is formed by applying polyvinyl carbazole by the casting method exhibits electroluminescence (Polymer, 24 (1983), 755). However, by the above method, it is quite difficult to obtain a thin and uniform film, and luminescence and film thickness in the above device are considered to be less than 1 cd/m.sup.2 and more than several microns, respectively.
It is reported that a device in which a thin conjugated polymer film is used as a light emitting layer, said thin conjugated polymer film being formed by the following method. A precursor polymer film is formed by the spin-coating method and heated to form the thin conjugated polymer film (WO 90/13148). However, by the spin-coating method, it is quite difficult to obtain thin and pinholeless film less than 0.1 .mu.m in thickness.
The light emitting device with large area uniform emission cannot be obtained by any of the above methods. In addition, the light emitting layer formed by the above method is contaminated with various impurities derived from a solution including the precursor polymer. The impurities easily diminish luminescence and efficiency of the EL device, and the device with high performance and long life cannot be obtained.
An EL device in which poly-silazane layer prepared by the spin-coating method in a solution of the polymer is used as a hole transporting layer is reported (U.S. Pat. No. 4,950,950). The electroluminescence device with high performance and long life cannot be obtained by any of the above methods because of the aforesaid defects.